The present invention relates generally to voltage regulators, and more particularly to control systems for switching voltage regulators.
Voltage regulators, such as DC to DC converters, are used to provide stable voltage sources for electronic systems. Efficient DC to DC converters are particularly needed for battery management in low power devices, such as laptop notebooks and cellular phones. Switching voltage regulators (or simply "switching regulators") are known to be an efficient type of DC to DC converter. The switching regulator generates an output voltage by converting an input DC voltage into a high frequency voltage, and filtering the high frequency input voltage to generate the output DC voltage. Specifically, the switching regulator includes a switch for alternately coupling and de-coupling an unregulated input DC voltage source, such as a battery, to a load, such as an integrated circuit. An output filter, typically including an inductor and a capacitor, is coupled between the input voltage source and the load to filter the output of the switch and thus provide the output DC voltage. The switch is typically controlled by a pulse modulator, such as a pulse width modulator or a pulse frequency modulator. A feedback system generates a control signal which controls the duty cycle of the pulse modulator in order to maintain the output voltage at a substantially uniform level.
In many conventional switching regulators, the control signal generated by the feedback circuit is a control voltage. The control voltage is compared to a ramp voltage, such as a sawtooth voltage waveform generated by a ramp generator. When the control voltage exceeds the ramp voltage, the switch is closed to connect the voltage source to the load, whereas if the control voltage is lower than the ramp voltage, the switch is opened to disconnect the voltage source from the load.
The gain of a pulse modulator is the ratio between the control voltage and the average output voltage. Thus, the gain, A, is approximately equal to: ##EQU1## where V.sub.OUT is the average output voltage and V.sub.CONTROL is the control voltage. If this gain is not constant, the feedback system will not be stable, the gain of the pulse modulator will vary, and the output voltage will not be substantially uniform.